Fabrication of a Miniaturized Room Temperature
Ionic Liquid Gas Sensor for Human Health and
Safety Monitoring
Xiaoyi Mu, Student Member, IEEE, †Zhe Wang, †Min Guo, †Xiangqun Zeng, Andrew J. Mason, Senior Member,
IEEE
Electrical and Computer Engineering, Michigan State Univ., East Lansing, MI, USA
†Chemistry Department, Oakland University, Rochester, MI, USA
A promising approach is to utilize an electrochemical
sensing methodology[4] with room temperature ionic
liquids (RTILs) as the sensing media[5]. When sensing
gases, electrochemical methods provide good selectivity
with low power consumption and wide dynamic range. The
instrumentation needed for such methods can readily be
implemented as a single microelectronics chip suitable for
portable/wearable systems[6-8]. RTILs are nonvolatile and
conductive compounds consisting entirely of ions. With
properties such as negligible vapor pressure, wide potential
windows, and high thermal stability, RTILs offer a
promising electrolyte for robust electrochemical gas sensors
that can operate in extreme conditions. In addition, unlike
conventional tin dioxide gas sensors that operate at over
200 °C[9], RTILs function at room temperature and thus
greatly reduce power demands by eliminating the need to
heat the sensor. With electrochemical measurement, RTILs
are capable of sensing a variety of gases, including oxygen,
combustible gases and ambient toxic gases [10-13].
Abstract— The growing potential impact of airborne
pollutants on human health and safety has escalated the
demand for sensors to monitor hazardous gases. Room
temperature ionic liquid (RTIL) gas sensors utilizing
electrochemical instrumentation demonstrate promising
sensitivity, selectivity, and miniaturization capabilities. This
paper introduces a microfabrication process that enables
miniaturized, rapid response, gas sensors to be realized using
RTIL interfaces on a permeable membrane substrate with
planar microfabricated electrodes. An RTIL sensor with a
2mm×2mm sensing area is described, and measured responses
to methane, a dangerous residential and occupational gas, and
sulfur dioxide, a common environmental pollutant, are shown.
The reported sensor structure and fabrication process enable
realization of sensor arrays for multi-gas monitoring in a low
power, miniaturized, wearable system platform.
Keywords – Gas sensor, planar electrode, electrochemical
impedance spectroscopy, room temperature ionic liquid
I.
INTRODUCTION
Exposure to air toxins and harmful gases is an ever
present concern for human health in modern society.
Airborne pollutants cause discomfort, illness, and even
death[1]. Similarly, as energy demands continue to
increase, exposure to explosive gases at home or at work
threaten human safety. For example, exposure to sulfur
dioxide (SO2) correlates to an increase in cardiopulmonary
mortality, and methane (CH4) leakage from stoves and
boilers in homes are sources of fires and explosions. As
these dangers to human health and safety increase, there is a
growing need for miniaturized, low power, multi-gas
monitoring systems suitable for individuals to wear and
capable of constantly examining the surrounding
environment. Although numerous efforts have been made to
develop gas sensors that can accurately measure air
pollutants[2, 3], a low-cost, low-power, real-time, wearable
microsystem for gas monitoring that can be widely
distributed for biomedical applications is still unavailable.
Our team has recently developed an RTIL-based
electrochemical gas sensor that demonstrates high
sensitivity and rapid response to methane[14]. This device
utilizes a structure of RTIL coating on metal electrodes over
a porous polytetrafluoroethylene (PTFE) membrane. This
paper reports a fabrication process and gas sensor structure
that enables significant miniaturization of the device .
Section II introduces the planar-electrodes-on-permeablemembrane (PEoPM) gas sensor structure. Section III
presents the microfabrication process for a miniaturized
PEoPM gas sensor. Finally test results of a miniaturized gas
sensor are shown in Section IV.
II.
SENSOR STRUCTURE AND FABRICATION OF MACRO
SENSOR
The main obstacle to response time in existing RTILbased electrochemical gas sensors[10-13] is the slow
diffusion of target gases from the RTIL surface to the
electrodes. As illustrated in Fig. 1(a), in the conventional
sensor structure, the electrodes are fabricated on a substrate
and RTIL is then coated on top of the electrodes. Because
To realize such a reliable multi-gas monitor in a
microsystem platform, a gas sensor array with fast response
time, low power consumption, and small footprint is needed.
140
machining (EDM) which only provides a resolution of
around 200µm. Thus the WE-CE gap could not be reduced
below 200µm, which required the electrodes to occupy a
large area to keep the gap resistance reasonable. To
overcome this significant limitation, the fabrication process
described in section III was developed.
III. FABRICATION OF A MINIATURIZED SENSOR
Microfabrication processes has been well developed and
widely used in the semiconductor industry. Using
photolithography followed by thermal metal deposition and
liftoff process, electrode patterns on smooth substrates can
easily be formed. Traditional photolithography has a
resolution on the order of 1µm and should be suitable for
fabricating miniaturized gas sensors. However, directly
applying traditional photolithography on porous PTFE
substrates, as desired here, introduces several problems that
require development of a new photolithography process for
porous substrates.
Fig. 1. (a) Conventional sensor structure: response time is slow due to
slow gas diffusion through RTIL; (b) electrodes-on-permeable-membrane
structure: response time is improved due to fast gas diffusion in the
permeable membrane.
the electrochemical reaction happens at the interface of the
RTIL and the electrodes, a response cannot be measured
until the gas analyte dissolves into the RTIL and reaches the
electrodes. A full-scale response to gas analyte is achieved
only after an equilibrium forms on the interface. The time
interval between the gas reaching the electrodes’ surface
and establishing equilibrium is primarily dependent upon
the gas diffusion speed in the RTIL. Due to RTIL’s high
viscosity, RTIL-based electrochemical gas sensors suffer
low response caused by the slow gas diffusion in the RTIL.
By attaching electrodes to a gas-permeable membrane[11]
or reducing the RTIL thickness[12], the distance between
the RTIL surface and the electrodes can be reduced, thus
improving response time. However, these approaches still
require the gas to diffuse across the RTIL layer, where
diffusion rates are slow. In contrast, the PEoPM structure
that bypasses the slow diffusion of gas across the RTIL[14]
is illustrated in Fig. 1(b). The electrodes are fabricated
directly on a gas-permeable membrane, allowing gas to
reach the electrodes/RTIL interface through the permeable
membrane, where diffusion is much faster than in the RTIL.
To simplify the sensor structure complexity and lower the
fabrication cost, working electrodes (WE), counter
electrodes (CE) and quasi-reference electrodes (RE) should
all be fabricated on the permeable membrane. Using the
PEoPM structure, a 5% methane response time of 5s was
measured compared to 103s in the case of the traditional
structure[14].
First, note that photoresist is generally spin-coated onto a
substrate that is vacuum-attached to the spinner. However,
porous PTFE is soft and cannot be reliably held by vacuum
because it is porous. A preliminary experiment
demonstrated that, when a PTFE sheet was put directly on
the spinner, significant photoresist would be sucked into the
PTFE and could not be removed during liftoff. To resolve
this problem, the porous PTFE was clamped to a glass
substrate, and the glass was placed on the spinner and held
by vacuum.
Second, because porous PTFE has a rough surface,
standard photoresist coatings of PTFE are not reliably
smooth enough for patterning. For example, with porous
PTFE having a 4μm pore size, around 4μm of surface
roughness can be expected. To resolve this problem, a thick
film photoresist was selected. Hoechst AZ4620 was spincoated at 2100rpm to create 10μm thick layer. Lithography
by 60s UV exposure and 300s developing in AZ300 MIF
developer was found to reliably remove the thick resist. The
profile scan by a Dektak3 Surface Profiler in Fig. 2
demonstrates that the photoresist AZ4620 covered the
rough surfaces of porous PTFE and that a good edge step,
suitable for liftoff, was formed.
In previous work, a macro-scale sensor occupying a
48mm2 sensing area was fabricated as follows. First, a
stainless steel mask was mounted against the porous PTFE
and then a gold film was deposited. After removing the
mask, patterned planar gold electrodes were formed. Finally,
the RTIL was coated on the electrodes to form the sensor.
Because RTILs are a highly resistive electrolyte, the sensor
electrodes were designed to minimize the electrolyte
resistance by interdigitating the WE and CE electrodes and
minimizing the gap between them. However, the stainless
steel hard mask was prepared by electric discharge
Third, note that preliminary experiments found metal
liftoff to be difficult, even after a gold-deposited sample
was soaked in acetone for several days. One possible reason
is that the photoresist became polymerized during the half
hour physical vapor deposition, where the temperature on
the sample is over 100°C. Because polymerized photoresist
cannot easily be dissolved in acetone, this would cause the
liftoff to failure. To solve this problem, note that exposure
to UV should prevent photoresist from polymerizing. Thus,
141
Fig .2. Profile scan by Dektak3 Surface Profiler. The photoresist AZ4620
covered the rough surface of porous PTFE and a good step was formed.
a 60s flood exposure was performed after developing the
photoresist and before metal deposition.
Finally, because of the uneven distribution of pores on
PTFE surface, the conductivity of metal traces were found
to be unreliable and in the worst case could result in entire
electrode fingers being disconnected, resulting in significant
device-to-device variation. To resolve this problem, one
possible solution is to increase the width of the electrodes
thereby reducing the chance of a discontinuity. However,
this approach conflicts with the goal to miniaturize the
device. To analyze the relation between device variation
and electrode width, 4 groups of 10mm-long gold traces
with different widths were fabricated. Each group had eight
identical elements, and resistance values were measured to
compare their average sheet resistance and standard
deviation as shown in Table I. As expected, the results
show that traces with larger widths have better consistency.
200µm width was chosen as a compromise between
reliability and the miniaturization goal.
Fig .3. Miniaturized RTIL-based PEoPM sensor fabrication process.
readily deposited in thin films and patterned in a planar
process. High-purity RTIL 1-butyl-1-methylpyrrolidinium
bis(trifluoromethylsulfonyl)imide ([C4mpy][NTf2]) was
chosen as the electrolyte due to its low viscosity and high
chemical stability.
The porous PTFE sheet was clamped on glass wafer and
spin-coated by AZ4620 photoresist. 5min soft bake at 95°C
was performed, followed by a 60s UV exposure through the
electrode pattern mask. After 300s developing in AZ300
MIF developer, the sample was flood exposed for 60s. Then
a 300nm-thick gold film was deposited on the porous PTFE
sheet using physical vapor deposition (PVD, Edward 360
thermal evaporator). After deposition, the sample was
soaked in acetone overnight and the gold electrodes pattern
was formed by liftoff. Finally, to add the RTIL interface,
the electrodes were then coated with 200 µm-thick
[C4mpy][NTf2] with a droplet process.
By addressing each of the challenges described above,
the RTIL gas sensor fabrication process was optimized to
achieve a miniaturized PEoPM structure. The fabrication
process is illustrated in Fig. 3. POREX® porous PTFE with
35% porosity and 4μm pore size (Zitex TM, Chemplast,
Incorporated, Wayne, New Jersey) was chosen as the
permeable membrane due to its excellent inertness and
hydrophobic response to the RTIL. Gold was chosen as the
electrode material because it is highly inert and can be
TABLE I
AVERAGE SHEET RESISTANCE AND STANDARD DEVIATION WITH
DIFFERENT WIDTH CONFIGURATIONS
standard
deviation
(%)
0.288
standard
deviation
(Ω)
0.044
0.323
0.041
12.7
400
0.327
0.034
10.3
800
0.306
0.023
7.5
width
(µm)
average
(Ω)
100
200
IV. RESULTS
Following the microfabrication process above, the
miniaturized PEoPM device shown in Fig. 4 was fabricated.
The electrode structure occupies a 2mm×2mm sensing area,
only 8% of the area in the macro-scale device. WE and CE
were interdigitated for impedance measurement with a 200
µm width and a 100µm gap, and an RE was included to
improve electrochemical stability. An o-ring was used to
15.2
142
separate the sensing electrode area from the connection
pads during testing.
fast path for gas to diffuse to the active sensor electrode
area. Process challenges were addressed and tradeoffs were
analyzed. Using CH4 and SO2 as an example gases relevant
to human health and safety, the sensor’s functionality was
demonstrated. The sensor achieves the goals of small size,
low power, low cost, and fast response. The reported
microfabrication process enables future realization of
miniaturized sensor arrays for wearable multi-gas
monitoring microsystems.
The fabricated sensor was placed in a gas chamber and
sealed with the o-ring. Electrochemical impedance
spectroscopy (EIS) tests were performed with a VersaStat
MC potentiostat (Princeton Applied Research, Oak ridge,
TN, U.S.A.) using a 10mV peak-to-peak sinusoidal signal.
To demonstrate the functionality of the microfabricated
PEoPM device, CH4 and SO2 were selected as example
pollutants. The sensor was exposed to CH4 and SO2
individually, and the impedance spectra were recorded. CH4
concentration was varied from 0 to 5% because 5% is the
lower explosive limit for CH4; and SO2 concentration was
varied from 0 to 5ppm because 5ppm is the permissible
exposure limit for SO2. Impedance amplitude values at 1Hz
were extracted to form the CH4 and SO2 calibration curves.
Fig. 5 plots the normalized difference between the response
and baseline (0% CH4/SO2) impedances, Δ|Z|, verses
CH4/SO2. The results demonstrate good sensitivity over the
range of interest. In fact, the CH4 sensitivity of the
miniaturized device is comparable to the macro-scale
device [14] even though it is more than 10× smaller. This
miniaturized sensor is thus well suited to measure multiple
gases in a wearable real-time gas monitoring microsystem.
ACKNOWLEDGMENT
This work was supported by the National Institute for
Occupational Safety and Health (NIOSH) under Grant
R01OH009644.
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[1]
[2]
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V. CONCLUSION
This paper introduced a microfabrication process for a
miniaturized RTIL-based gas sensor featuring a planarelectrode-on-permeable-membrane structure that provides a
[6]
[7]
[8]
[9]
Δ|Z|/|Z|(%)
Fig. 4. Photograph of fabricated miniaturized PEoPM device. The WE
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80%
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0%
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0
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at 1Hz.
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